It is all due to a strike against parts supplier American Axle and Manufacturing Holdings Inc. by the United Auto Workers union that started early Tuesday morning. Detroit-based American Axle makes axles and other parts for the trucks, and GM is running out of the parts, Wickham said.

He would not say if the strike could lead to more GM plant closures, but industry analysts have said American Axle also makes components for GM's large sport utility vehicles.

"We are monitoring supply to other facilities," Wickham said.

Factories in Janesville, Wis., and Arlington, Texas, that make the large SUVs still were operating today. Their vehicles include the Chevrolet Tahoe and Suburban, GMC Yukon and Cadillac Escalade.

GM also still was making pickups and large SUVs at a factory in Silao, Mexico, and it makes a small number of pickups at a plant in Toluca, Mexico.

About 3,600 workers represented by the UAW at five American Axle plants in Michigan and New York walked off the job in a contract dispute. As of Thursday night, no talks had been scheduled.

American Axle had been using stockpiled parts to keep supplying GM and its other customers, although it appears those supplies are running out. The company makes axles, drive shafts and stabilizer bars.

GM accounts for about 80 percent of American Axle's business, with 10 percent going to Chrysler LLC and the rest to other automakers.